识别两个不同列中具有两个相同值的记录？

Question

我有一个 table 的学生 ID、专业 1、专业 2 和辅修。我想确定那些为同一专业注册两次的学生的记录。我需要一个函数来 select 只有那些拥有相同专业的人，例如 Major1 和 Major2 列中的 "FIN"。我到目前为止：

CREATE VIEW A5T5 AS
SELECT (firstname || ' ' || lastname)"FullName", StudentID AS "StudentID", Major1 as "DoubleDipping"
FROM A5
Group by Major1, Major2 ?????
Having count ?????
ORDER BY Major,LastName,FirstName;

Answer 1

为什么不这么简单：

SELECT 名字，姓氏 FROM A5 WHERE Major1 = Major2

Answer 2

我认为你让这比现在更难了。如果我正确理解了您的问题，则以下查询应该可以为您提供所需的信息：

SELECT (firstname || ' ' || lastname) AS "FullName",
       StudentID AS "StudentID",
       Major1 as "DoubleDipping"
  FROM A5
  WHERE MAJOR1 = MAJOR2

如果您确实需要将其作为函数，return 结果集的常用方法是 return 打开 SYS_REFCURSOR 调用者随后负责关闭。例如：

CREATE OR REPLACE FUNCTION DOUBLE_DIPPING_STUDENTS
    RETURN SYS_REFCURSOR
IS
  csr SYS_REFCURSOR;
BEGIN
  OPEN csr FOR SELECT (firstname || ' ' || lastname) AS "FullName",
                      StudentID AS "StudentID",
                      Major1 as "DoubleDipping"
                 FROM A5
                 WHERE MAJOR1 = MAJOR2;

  RETURN csr;
END DOUBLE_DIPPING_STUDENTS;

上述函数可以从 PL/SQL 块调用为：

DECLARE
  csr       SYS_REFCURSOR;
  strNAME   VARCHAR2(2000);
  nID       A5.STUDENTID%TYPE;
  strMAJOR  A5.MAJOR1%TYPE;
BEGIN
  csr := DOUBLE_DIPPING_STUDENTS;

  LOOP
    FETCH csr
      INTO NAME, ID, MAJOR;

    WHEN csr%NOTFOUND THEN EXIT;

    DBMS_OUTPUT.PUT_LINE(strNAME || '  ' || nID || '  ' || strMAJOR);
  END LOOP;

  CLOSE csr;
END;

识别两个不同列中具有两个相同值的记录？

Identify records with two identical values in two different columns?

sql

oracle

duplicates